The telecoms giant which owns EE, was found to have overcharged almost 6,000 EE mobile customers around £42,000 for making calls to the 118 service to obtain numbers for a person or business.

Ofcom, the regulator, introduced a price cap on 118 phone numbers in April 2019, but BT failed to implement the change immediately for its EE customer base.

BT was able to correct most bills before EE customers paid them but it meant that around £10,000 was overcharged and paid by the users.

All customers have been refunded but Ofcom has fined BT £245,000 (a 30% discount for admitting its failings) for “a serious breach of rules”, particularly considering the vulnerable people who use the number search service.

While use of directory enquiries has fallen in recent years, many people, particularly older people and those without internet access, still rely on the service to obtain the numbers they need.

The money raised from the fine will be passed on to HM Treasury. The 118 price cap stands at £3.65 per 90 seconds. Before the cap, callers were charged almost £20.

Ernest Doku, mobiles expert at​ comparison site uSwitch, said: “Unfortunately the error is most likely to have affected older and vulnerable people, who are more inclined to use 118 numbers.

“BT, which owns EE, did realise its mistake and corrected most bills before they were paid by EE customers – but not before customers had to pay out £10,000 unnecessarily.

“All customers should have been refunded now, but if you think you have been affected by a similar mistake, contact your mobile company in the first instance.

“If you are unhappy with their response, you can take your complaint further by getting in touch with the appropriate ombudsman for your mobile provider.”

An EE spokesperson, said: “We apologise for any inconvenience that this mistake caused to a very small number of our customers. Customers that were affected were overcharged by less than £5 on average and we quickly contacted and refunded every one of them. We have taken this very seriously and have reviewed and updated our procedures to ensure it does not happen again.’’